Apparently, the new Windows 9 Start menu is capable of matching the different backgrounds of the wallpaper that you are utilizing at the moment by changing its color, just like how a chameleon would. A very good example would be the image on top, where the Start menu alters its color depending on the kind of background chosen.
This might be seen by some as a tiny personalization step for the operating system, but it does have its appeal as not everyone loves the standard set up, and with the amount of customization going on these days in our smartphone and tablet interfaces, it pays for Microsoft to throw in something like that in Windows 9.